Semiconductors such as titanium oxide, due to their photocatalytic activity, have antibacterial properties, hydrophilicity, stain resistance, anti-fogging properties, gas decomposing properties, deodorizing properties, water-treating capabilities, energy conversion properties and other properties, and thus are used in a variety of fields.
In particular, photoelectric conversion devices, such as solar cells, that utilize the energy conversion properties of titanium oxide and other semiconductors are attracting attention as a means for producing electrical energy without adverse effects on the global environment.
When using titanium oxide or other semiconductors as photocatalysts, it is preferable to form them into porous films on substrates.
With regard to processes for forming a porous film of semiconductor, such as titanium oxide, Japanese Unexamined Patent Publication No. 1998-212120 discloses a process for forming a porous titanium oxide film, comprising applying a dispersion of titanium oxide particles in a glyme solvent, onto a glass, metal, ceramic or like substrate by spray coating, dip coating or like process, and then baking the coating at 200 to 800° C.
Further, Japanese Unexamined Patent Publication No. 2002-145615 discloses a process for forming a porous titanium oxide film on a substrate, comprising intermittently spraying a starting solution obtained by adding hydrogen peroxide or aluminum acetylacetonate to a titanium oxide precursor, onto a substrate such as glass maintained at a high temperature of 350° C. or 500° C. so as to thermally decompose the titanium oxide precursor to titanium oxide.
However, these processes have a drawback in that they involve heating at a high temperature of 200° C. or more to form a porous titanium oxide film, and therefore are not applicable to thermoplastic resin substrates that deform or degrade at temperatures of 200° C. or higher.
Japanese Unexamined Patent Publication No. 1999-204152 discloses a process comprising applying a dispersion of metal oxide particles in a high polymer material solution to a high polymer film provided with a conductive layer, and drying the dispersion at a temperature of 200° C. or lower.
This process employs a relatively low heating temperature, and thus is capable of forming a titanium oxide film on thermoplastic resin substrates. In this process, however, since the metal oxide particles are dispersed in a high polymer material solution, the obtained titanium oxide film has good adhesion but is unlikely to be porous. Thus, the process has a drawback in that the resulting film has inferior energy conversion properties, such as photoelectric conversion efficiency.